as in treacherous
formal
causing harm in a way that is gradual or not easily noticed
Most people with this insidious disease have no idea that they are infected. an insidious enemy
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Recent Examples of insidious
Another of measles’ worst and most insidious outcomes is subacute sclerosing panencephalitis (SSPE), an untreatable neurodegenerative condition that can take nearly a decade to manifest.
—
Katherine J. Wu, The Atlantic, 24 Feb. 2026
Political discussions on social media are often dominated by competing attributions of more and more insidious motives to people on the other side.
—Mark Schroeder, The Conversation, 20 Feb. 2026
Here, the joyous and the boisterous contrast the dark and insidious, allowing viewers to both be delighted by colorful street scenes and celebrations while the rot of fear creeps in, as the details surrounding our main character (Wagner Moura) and his predicament are revealed.
—Megan Spurrell, Condé Nast Traveler, 11 Feb. 2026
The abstracting reached its pinnacle (nadir) with an insidious Alexa ad featuring Chris Hemsworth and wife Elsa Pataky.
—Steven Zeitchik, HollywoodReporter, 9 Feb. 2026
See All Example Sentences for insidious
Katherine J. Wu, The Atlantic, 24 Feb. 2026
Political discussions on social media are often dominated by competing attributions of more and more insidious motives to people on the other side.
—Mark Schroeder, The Conversation, 20 Feb. 2026
Here, the joyous and the boisterous contrast the dark and insidious, allowing viewers to both be delighted by colorful street scenes and celebrations while the rot of fear creeps in, as the details surrounding our main character (Wagner Moura) and his predicament are revealed.
—Megan Spurrell, Condé Nast Traveler, 11 Feb. 2026
The abstracting reached its pinnacle (nadir) with an insidious Alexa ad featuring Chris Hemsworth and wife Elsa Pataky.
—Steven Zeitchik, HollywoodReporter, 9 Feb. 2026
See All Example Sentences for insidious
Recent Examples of Synonyms for insidious
Adjective
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But over this rocky, treacherous stretch of coastline, its beam is shining once again.—
Los Angeles Times,
Los Angeles Times,
11 Mar. 2026
-
The winner is expected to reach the finish line around March 17 after traveling over two mountain ranges, along the frozen Yukon River and on the treacherous ice of the Bering Sea on Alaska’s western coast.—
ABC News,
ABC News,
10 Mar. 2026
Adjective
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Among California’s most upscale neighborhoods, a subtle reshuffling has taken place in recent decades, with the highest home values migrating from Northern California to Southern California.—
Terry Castleman,
Los Angeles Times,
12 Mar. 2026
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Key Takeaways The symptoms of transthyretin amyloid cardiomyopathy (ATTR-CM)—a disease of the heart muscle—are often subtle and can mimic those of other heart conditions, especially in the early stages.—
Colleen Doherty,
Verywell Health,
11 Mar. 2026
Adjective
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Chapman is charged with making a false statement during a firearm purchase and engaging in the business of firearms dealing without a license.—
Arkansas Online,
Arkansas Online,
14 Mar. 2026
-
Thomas Edward Manfredi, 33, is charged with leaving the scene of a crash involving serious bodily injury and making a false police report, according to an arrest warrant.—
Shira Moolten,
Sun Sentinel,
13 Mar. 2026
Adjective
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In the short term, the company focuses on immediate relief from rising annual maintenance fees, open loans, and the financial fallout from deceptive sales practices.—
Malana VanTyler,
USA Today,
9 Mar. 2026
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Tenet had sued Leapfrog last year over poor grades given to its Palm Beach Network hospitals, calling the grading process unfair and deceptive.—
Cindy Krischer Goodman,
Sun Sentinel,
9 Mar. 2026
Adjective
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For those trying to read Iran’s future, the lesson of 1953 is not just that foreign powers can be perfidious.—
Bobby Ghosh,
Time,
5 Mar. 2026
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Observing human behavior confirms that for some among us, the perfidious lust for unbridled power and the imposition of cruelty in its quest know no bounds and are bereft of human decency.—
A.O. Scott,
New York Times,
3 Feb. 2026
Adjective
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The statement came after Bloomberg reported that Oracle and OpenAI had dropped plans to expand the site, though Oracle said media reports regarding Abilene were incorrect.—
Jordan Novet,
CNBC,
10 Mar. 2026
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He was charged in 2013 with tax fraud for filing an incorrect return tied to a Ponzi scheme operated by another man.—
Nick Penzenstadler,
USA Today,
10 Mar. 2026
Adjective
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This story has been updated to correct wrong information from the Monroe County Sheriff’s Office.—
David Goodhue,
Miami Herald,
14 Mar. 2026
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So don't go off topic, don't impersonate anyone, and don't get your facts wrong.—
Damon Wise,
Deadline,
14 Mar. 2026
Cite this Entry
“Insidious.” Merriam-Webster.com Thesaurus, Merriam-Webster, https://www.merriam-webster.com/thesaurus/insidious. Accessed 19 Mar. 2026.
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